At the center and to the right is a medullary carcinoma of thyroid, which is much more cellular than the adjacent normal thyroid follicles at the left. At the far right is pink hyaline material with the appearance of amyloid. These neoplasms are derived from the thyroid "C" cells and, therefore, can have neuroendocrine features such as secretion of calcitonin or other hormones.

C-cell hyperplasia is a precursor lesion to MTC, and it appears as aggregates of cells with abundant pink to clear cytoplasm and granular chromatin. It is common in the 25% of MTCs that are hereditary from germline RET gene mutations, and occasionally is observed in sporadic MTCs.